x86 memory detection

Hi, How does the kernel detects the amount of available System RAM in the system under x86 architecture?. Under 2.6.x kernels, 1. Does it use BIOS interrupt 15 for determining the RAM OR 2. Does the boot loader fills the e820_map as explained inDocumentation/x86/boot.txt & Documentation/x86/zero-page.txt and the kernel uses in information OR 3. Is there any other mechanism (if so, in which file under kernel tree I can find this detection code) Thanks, Arun